1. Field of the Invention
The present invention relates to a dielectric ceramic and a capacitor using the same. More particularly, the present invention relates to a pulse-generating capacitor which is used as a starter of a discharge lamp, inter alia, a high-pressure vapor discharge lamp which houses a starter inside an outer bulb of the lamp.
2. Background Art
An ordinary commercial power supply is insufficient for starting a high-pressure vapor discharge lamp such as a high-pressure sodium lamp, and therefore, a high voltage pulse must be applied to the lamp. There has come into wide use a high-pressure vapor discharge lamp in which a starter for generating a high voltage pulse is built into an outer bulb and which is used in combination with a ballast for ordinary high-pressure mercury lamps. Such a high-pressure vapor discharge lamp has a basic structure such that a capacitor formed from a non-linear dielectric ceramic is connected in parallel to an arc tube, into which a semiconductor switch (SSS) is incorporated in order to generate a high voltage pulse. The thus generated high voltage pulse is applied to the arc tube together with a power source voltage in order to start the discharge lamp.
As a capacitor serving as means for stably generating such a high voltage pulse, a non-linear dielectric ceramic capacitor made of a barium titanate polycrystalline substance has been used.
As shown in FIG. 1, such a non-linear dielectric ceramic capacitor exhibits a D-E hysteresis in which dielectric displacement (D) sharply changes with voltage (E), and when a voltage greater than the coercive field of the dielectric ceramic capacitor is applied to the capacitor, the amount of charge quickly reaches a saturation level in the vicinity of the polarization reversal voltage. Variation in current at this time causes a change in the ballast, so that a pulse of a high voltage corresponding to xe2x88x92Lxc2x7di/dt is generated due to the inductance of the ballast.
A non-linear dielectric ceramic capacitor used for a high-pressure vapor discharge lamp such as a high-pressure sodium lamp is required to have a steep D-E hysteresis which is stable over a wide temperature range. Non-linear dielectric ceramic capacitors that meet such a requirement are disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 63-221504, 63- 221505, 1-136323, and 1-136324.
In general, the interior of the outer bulb of a high-pressure vapor discharge lamp such as a high pressure sodium lamp is maintained at a high vacuum of 1xc3x9710xe2x88x925 torr, and during a period in which light is on, the interior of the outer bulb is exposed to high temperature (300xc2x0 C.) and high vacuum (1xc3x9710xe2x88x925 torr). Further, a barium getter is disposed inside the outer bulb in order to cause adsorption of oxygen generated when the discharge lamp is operated so that the degree of vacuum inside the outer bulb is maintained. However, when the discharge lamp is operated for a long period of time, the interior of the outer bulb becomes a reducing atmosphere due to hydrogen adsorbed onto the arc tube, a metal support for the arc tube, the glass or a like member that constitutes the outer bulb, or due to hydrogen generated as a result of decomposition of adsorbed water. Therefore, when a high-pressure vapor discharge lamp is used for a long period of time in a state in which a non-linear dielectric ceramic capacitor disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 63-221504, 63-221505, 1-136323, or 1-136324 is built into the outer bulb to be used for pulse generation, the dielectric ceramic is reduced, and thus the insulating resistance decreases resulting in the problem that the voltage of generated pulses decreases, or no pulse is generated, with the result that the discharge lamp does not start.
In order to solve this problem, there have been proposed the measure disclosed in Japanese Patent Application Laid-Open (kokai) No. 60-52006 in which a dielectric ceramic capacitor is completely coated with inorganic glass except for its electricity supply portions, and a measure disclosed in Japanese Patent Application Laid-Open (kokai) No. 4-34832 in which a hydrogen adsorption getter is disposed inside the outer bulb. However, these measures cannot suppress deterioration of the non-linear dielectric ceramic capacitor. In addition, the structure of the capacitor for pulse generation and the discharge lamp becomes complex, resulting in increased cost. Further, if the dielectric ceramic capacitor is completely coated with inorganic glass as in the invention of Japanese Patent Application Laid-Open (kokai) No. 60-52006, the D-E hysteresis characteristics of the dielectric ceramic capacitor deteriorate due to the glass so that high voltage pluses cannot be generated in some cases.
In view of the foregoing, an object of the present invention is to provide an inexpensive capacitor for pulse generation (hereinafter may be referred to as a pulse-generating capacitor) whose characteristics do not deteriorate even when the capacitor is used in a high temperature, high vacuum, reducing atmosphere, and which enables generation of high-voltage pulses over a wide temperature range.
In one aspect of the present invention, there is provided a pulse-generating capacitor which is used inside an outer bulb of a high-pressure vapor discharge lamp, said capacitor comprising a dielectric element constituted of a non-linear dielectric ceramic which exhibits resistance against reduction.
Preferably, the non-linear dielectric ceramic which exhibits resistance against reduction comprises a polycrystalline substance containing barium titanate as a primary component, and when the polycrystalline substance is represented by the following formula:
(1xe2x88x92axe2x88x92b)ABO3+aM+bR
wherein ABO3 is the barium titanate component and represents a perovskite structure, M is an oxide of at least one element selected from the group consisting of Mn, Ni and Co, R is an oxide of at least one element selected from the group consisting of La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb, and a and b represent mole fractions,
A, B, a, and b satisfy the following relationships:
xe2x80x831.000xe2x89xa6A/Bxe2x89xa61.006 (mole ratio)
0.3xe2x89xa6b/axe2x89xa63
0.0015xe2x89xa6axe2x89xa60.0050
and
0.0015xe2x89xa6bxe2x89xa60.0050
and a total content of M and R represented by Ad satisfies
0.3 less than Adxe2x89xa61.0 (wt. %).
These ranges will be collectively referred to as a first preferable range.
Also preferably, the non-linear dielectric ceramic further comprises an oxide containing Si as a main component in an amount of about 0.005-0.1 parts by weight with respect to 100 parts by weight of the polycrystalline substance. This range for the Si-containing oxide and the aforementioned ranges, i.e., 1.000xe2x89xa6A/Bxe2x89xa61.006 (mole ratio), 0.3xe2x89xa6b/axe2x89xa63, 0.0015xe2x89xa6axe2x89xa60.0050, 0.0015xe2x89xa6bxe2x89xa60.0050, 0.3 less than Adxe2x89xa61.0 (wt. %), will hereafter be collectively referred to as a second preferable range.
Also preferably, the above-described ABO3 when represented by {(Ba1xe2x88x92xxe2x88x92yxe2x88x92zSrxCayMgz)O}m(Ti1xe2x88x92xcfx86xe2x88x92pZrxcfx86Hfp)O2 is such that x, y, z, xcfx86, p, and m satisfy the following relationships:
0xe2x89xa6xxe2x89xa60.05
0xe2x89xa6yxe2x89xa60.02
0xe2x89xa6zxe2x89xa60.005
0xe2x89xa6xcfx86xe2x89xa60.12
0xe2x89xa6pxe2x89xa60.12
0.0035xe2x89xa6xcfx86+pxe2x89xa60.12
1.000xe2x89xa6mxe2x89xa61.006.
These ranges will be collectively referred to as a third preferable range.
In another aspect of the present invention, there is provided a dielectric ceramic which comprises a polycrystalline substance containing barium titanate as a primary component, wherein when the polycrystalline substance is represented by the following formula:
(1xe2x88x92axe2x88x92b)ABO3+aM+bR
wherein ABO3 is the barium titanate component and represents a perovskite structure, M is an oxide of at least one element selected from the group consisting of Mn, Ni and Co, R is an oxide of at least one element selected from the group consisting of La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb, and a and b represent mole fractions, A, B, a, and b satisfy the following relationships:
1.000xe2x89xa6A/Bxe2x89xa61.006 (mole ratio)
0.3xe2x89xa6b/axe2x89xa63
0.0015xe2x89xa6axe2x89xa60.0050
and
0.0015xe2x89xa6bxe2x89xa60.0050
and a total content of M and R represented by Ad satisfies
0.3 less than Adxe2x89xa61.0 (wt. %).
In yet another aspect of the present invention, there is provided a dielectric ceramic which comprises a polycrystalline substance containing barium titanate as a primary component, and when the polycrystalline substance is represented by the following formula:
(1xe2x88x92axe2x88x92b)ABO3+aM+bR
wherein ABO3 is the barium titanate component and represents a perovskite structure, M is an oxide of at least one element selected from the group consisting of Mn, Ni and Co, R is an oxide of at least one element selected from the group consisting of La, Ce, Nd, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er and Yb, and a and b represent mole fractions, A, B, a, and b satisfy the following relationships:
1.000xe2x89xa6A/Bxe2x89xa61.006 (mole ratio)
0.3xe2x89xa6b/axe2x89xa63
0.0015xe2x89xa6axe2x89xa60.0050
and
0.0015xe2x89xa6bxe2x89xa60.0050
and a total content of M and R represented by Ad satisfies
0.3 less than Adxe2x89xa61.0 (wt. %),
and the dielectric ceramic further comprises an oxide containing Si as a main component in an amount of about 0.005-0.1 parts by weight with respect to 100 parts by weight of the polycrystalline substance.
Preferably, the above-described ABO3 when represented by {(Ba1xe2x88x92xxe2x88x92yxe2x88x92zSrxCayMgz)O}m(Ti1xe2x88x92xcfx86xe2x88x92pZrxcfx86Hfp)O2 that x, y, z, xcfx86, p, and m satisfy the following relationships:
0xe2x89xa6xxe2x89xa60.05
0xe2x89xa6yxe2x89xa60.02
0xe2x89xa6zxe2x89xa60.005
0xe2x89xa6xcfx86xe2x89xa60.12
0xe2x89xa6pxe2x89xa60.12
0.0035xe2x89xa6xcfx86+pxe2x89xa60.12
1.000xe2x89xa6mxe2x89xa61.006.
In a still further aspect of the present invention, there is provided a high-pressure metal vapor discharge lamp using as a starter a capacitor of the present invention.